1. Field of the Invention
The invention is directed to a single mode optical waveguide fiber for use in telecommunication systems and more particularly, a waveguide fiber which reduces non-linear dispersion effects, combines bend resistance, low attenuation, and large effective area features desired for underground and undersea applications.
2. Technical Background
Optical amplifier technology and wavelength division multiplexing techniques are typically required in telecommunication systems that require high power transmissions for long distances. The definition of high power and long distances is meaningful only in the context of a particular telecommunication system wherein a bit rate, a bit error rate, a multiplexing scheme, and perhaps optical amplifiers are specified. There are additional factors, known to those skilled in the art, which have impacted upon the meaning of high power and long distance. However, for most purposes, high power is an optical power greater than about 10 mW. In some applications, power levels of 1 mW or less are still sensitive to non-linear effects, so that the effective area is still an important consideration in such lower power systems. A long distance is one in which the distance between electronic regenerators can be in excess of 100 km. The regenerators are to be distinguished from repeaters which make use of optical amplifiers. Repeater spacing, especially in high data density systems, can be less than half the regenerator spacing. To provide a suitable waveguide for a multiplex transmission, the total dispersion should be low, but not zero, and have a low slope over the window of operating wavelength.
Generally, an optical waveguide fiber having a large effective area, Aeff, reduces non-linear optical effects, including self phase modulation, four wave mixing, cross phase modulation, and non-linear scattering processes, all of which can cause degradation of signals in high powered systems. A waveguide fiber having a segmented core can generally provide a large effective area while limiting the non-linear optical effects.
The mathematical description of these non-linear effects includes the ratio, P/Aeff, where P is the optical power. For example, a non-linear optical effect can be described by an equation containing a term, exp [Pxc3x97Leff/Aeff], where Leff is effective length. Thus, an increase in Aeff produces a decrease in the non-linear contribution to the degradation of a light signal. A core having multiple segments each characterized by a refractive index profile, a relative index, and a radius, meets many of the desired functional properties.
The requirement in the telecommunication industry for greater information capacity over long distances, without electronic signal regeneration, has led to a reevaluation of single mode fiber index profile design. The focus of this reevaluation has been to provide optical waveguides which:
reduce non-linear effects such as those noted above;
are optimized for the lower attenuation operating wavelength range around 1550 nm;
are compatible with optical amplifiers; and, retain the desirable properties of waveguides such as high strength, fatigue resistance, and bending resistance.
Communication systems which typically require one gigabyte, and higher, transmission rates, together with regenerators spacing in excess of 100 km, typically make use of optical amplifier technology or wavelength division multiplexing techniques. Thus waveguide fiber manufacturers have designed waveguides which are less susceptible to non-linear effects induced by higher power signals or by four wave mixing, which can occur in multiplexing systems. A suitable waveguide fiber must have low linear dispersion and low attenuation as well. In addition, the waveguide fiber must display these properties over a particular extended wavelength range in order to accommodate wavelength division multiplexing used for multiple channel transmission.
Submarine systems, which are normally several thousand kilometers in length, pose more stringent requirements on waveguide fiber profile designs. In such systems the non-linear effect modulational instability can cause significant accumulated signal distortion and thus system performance degradation. To overcome this problem, in addition to the optical waveguide fiber properties listed above, the dispersion of the waveguide over the wavelength window of operation is preferably negative.
Waveguide designs which also are relatively easy to manufacture and which permit management of dispersion are favored, because of their low cost and added flexibility. The designs described herein are well suited to a dispersion managing strategy in which the waveguide dispersion is varied along the waveguide fiber length to toggle the total dispersion between positive and negative values.
U.S. Pat. No. 5,781,684, incorporated herein by reference as though fully set forth in its entirety, discloses and describes a segmented core waveguide fiber having a large effective area. A feature of the segmented core of the waveguide fiber disclosed in the ""684 patent is that it at least one of the segments has a negative or a relative refractive index. The present application discloses and describes segmented core waveguide fibers that provide a unique set of functional properties.
This invention meets the need for a single mode optical waveguide fiber that offers the benefits of a relatively large effective area together with a substantially flat, negative, dispersion over an extended operating range.
The invention relates to a single mode optical waveguide fiber including a segmented core. Each of the segments is described by a refractive index profile, a relative refractive index percent, and an inner and outer radius. In two embodiments of the present invention disclosed herein, at least one of the segments has a negative relative refractive index percent. The optical waveguide fiber further includes a clad layer surrounding and in contact with the core, and having a refractive index profile.
In a preferred embodiment, the index profiles are further selected to provide a dispersion slope of less than about 0.1 ps/nm2-km. A further embodiment has a dispersion slope of less than 0.08 ps/nm2-km while maintaining a bend induced loss in the pin array test less than about 9 dB and preferably less than 8.8 dB.
In addition, embodiments having induced attenuation loss due to lateral load bending less than 0.25 dB/km and preferably less than 0.208 dB/km are disclosed and described.